The only general points I'd make for a view in that form are:
1) Does it really need UNION (which does an implicit distinct) or will UNION ALL suffice (which doesn't do the distinct)?
2) Is it possible to combine 2 or more of the existing selects you've unioned into a single select?

For any other suggestions we would need the information detailed in the link above.

Probably but I really wouldn't.
At some point you may need to query that view in a way where the hint is problematic.
If you want to use hints put them in the queries against the view, not in the view itself.

Also, I generally regard hints as the last resort, I'd see if you can speed it up without them first.

Then you should probably check - I imagine it'll be a fair bit longer than 10 seconds.
For a lot queries oracle can start returning data to the caller before it finishes finding all the rows that match the where clause.
If you use union, with it's implicit distinct, then it can't. It has to find all the matching rows before it can do the distinct. So it has to finish doing all it's work before you see any results.

UNION ALL will be faster than UNION, but not to the tune of 4 hours. Better see how long it takes now, you may well still have work to do.